1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a computer-readable medium, which are suitable to execute image processing on each frame of a moving image.
2. Description of the Related Art
Recently, with the progress of digital technology, digital processing has been generally performed even on an image obtained with, e.g., medical X-ray fluoroscopy. In particular, a two-dimensional X-ray sensor capable of outputting an X-ray image as digital data has also been developed instead of the known radiography using films for X-ray diagnosis. Further, digital image processing, such as a gradation process, has become essential in an X-ray fluoroscopic apparatus using the two-dimensional X-ray sensor.
In the X-ray fluoroscopy, AEC (Auto Exposure Control) is performed by detecting a dose of X-rays passing through a subject and by controlling the X-ray dose to just a preset value. The AEC includes taking out feature information, such as an average value of an X-ray fluoroscopic image which is obtained by using pulse-like X-rays emitted from an X-ray generator, comparing a level of the feature information with a reference value, and controlling X-ray emission conditions so that the desired exposure is obtained. The X-ray emission conditions include the tube voltage and the tube current of the X-ray generator, the width of an X-ray pulse, etc.
In the image processing and the AEC which are performed in the X-ray fluoroscopic apparatus, a captured image is subjected to a process of extracting, from the image, a region of interest (target region) corresponding to an anatomic structure of a human body, which is the most important image section particularly from the diagnostic point of view, for the purpose of properly displaying the target region. Therefore, the X-ray fluoroscopic apparatus is required to acquire, from the extracted target region, feature information used for the image processing and the AEC.
The target region differs depending on the location of the radiographic subject and the purpose of radiography. In stomach fluoroscopy using barium, for example, a stomach wall is the target region to detect a polyp erupted on the stomach wall. As other examples, in taking a moving image of the chest, a lung field region is the target region, and in radiography with cardiac catheterization, the distal end of a catheter and a surrounding region thereof provide the target region.
When limiting an irradiation field region by a collimator mounted to the X-ray generator, if a region other than the irradiation field region is included in the target region, such a region impedes a process of acquiring proper feature information, and therefore it should be excluded from the target region. Similarly, if the target region includes a through-exposure region where X-rays directly enter the two-dimensional X-ray sensor without passing through a subject, or a region of, e.g., a metal piece where X-ray absorptance greatly differs from the subject, those regions impede the process of acquiring proper feature information, and therefore they should also be excluded from the target region.
A method of extracting the target region from an image has hitherto been widely practiced by a threshold process of setting a threshold for discriminating the target region and other regions, and extracting the target region on the basis of the threshold, or by an edge extraction process of extracting a contour shape of an object on the basis of a shading distribution shape of the image.
For example, Japanese Patent Laid-Open No. 2001-351101 discloses a method of executing gray level correction of a radiological image on the basis of image information that is acquired from the density of a subject region in the radiological image. More specifically, in Japanese Patent Laid-Open No. 2001-351101, the feature information regarding the subject region in the image can be stably extracted by removing a through-exposure region and by extracting image component information corresponding to, e.g., soft tissues of the subject. According to Japanese Patent Laid-Open No. 2001-351101, therefore, effective image processing can be performed, for example, even when a maximum pixel density value in the subject region in the radiological image is smaller than a predetermined pixel density value.
Japanese Patent Laid-Open No. 2005-218581 discloses a method of extracting an irradiation field region for optimizing an image processing parameter. According to Japanese Patent Laid-Open No. 2005-218581, an irradiation field candidate region is first obtained by giving a score to the likelihood of an irradiation field edge from a pattern of a target pixel and surrounding pixels for adaptation to a circular, polygonal or other collimator shape. Then, regarding the obtained irradiation field candidate region, shape feature information indicating, e.g., a degree of circularity is obtained to determine a shape of the irradiation field candidate region, and an irradiation field region is extracted according to an irradiation field recognition algorithm specific to the determined shape. At that time, accuracy is increased by employing, as the algorithm specific to the shape, a straight-line detection process, such as Hough transform, for a polygonal shape, or template matching with a circle for a circular shape.
Japanese Patent Laid-Open No. 2003-250789 discloses a method of, in fluoroscopy to produce images at a relatively slow frame rate of 3 to 5 frames per second (fps), extracting a target region for use in acquiring feature information in order to properly execute processing with respect to at least one of the AEC and image density conversion. According to the method disclosed in Japanese Patent Laid-Open No. 2003-250789, a quadratic differential is executed on image data in upward, downward, rightward and leftward directions to obtain, as a line (boundary line) circumscribing an irradiation field, a position where a differential value is maximized in each direction, and a process of extracting the target region is executed on the basis of the obtained line circumscribing the irradiation field. In Japanese Patent Laid-Open No. 2003-250789, the process of extracting the target region is executed by changing an extraction algorithm for each target region in response to information representing the location of a radiographic subject or to request information.
However, the algorithm for extracting the target region is generally complicated. Particularly, in an image processing apparatus for processing a large amount of images, such as presented by an X-ray fluoroscopic apparatus, it is very difficult to extract the target region with high accuracy during a time from X-ray emission to display of the image at a required high frame rate (e.g., 25 to 30 fps).
Thus, the above-described related techniques may be suitable for extracting the target region from an image obtained in the case of taking a still image or performing fluoroscopy at a relatively slow frame rate, but they have a difficulty in properly extracting the target region from a moving image captured at a high frame rate.